The bold assertion by proponents of SENS (namely Aubrey de Grey) that "SENS is a practical, foreseeable approach to curing aging" has stirred considerable controversy among gerontologists. The reformulation of this assertion into a testable hypothesis will not only require concise definitions for the somewhat subjective terms "practical", "foreseeable", and "curing", it will require a precise definition of the term "aging". To facilitate proper experimental design, this definition must focus on the nature of aging itself, not its causes or consequences. For example, although curing aging will likely lead to increased longevity, increased longevity does not necessarily mean that aging has been cured. Aging in mammals is a process that begins early in adult life and continues, more or less, steadily thereafter until death. It is manifested by a decline in the functional capacity (or, more precisely, reserve capacity) of a variety of vital physiological systems leading to increasing risk of morbidity and mortality over time. Aging, however, cannot be measured by simply monitoring morbidity or mortality exclusively (or even both together). Aging can only be measured by monitoring the decline of global functional capacity itself. Thus, the author will propose an operational definition of aging. This definition will be expressed as a rate function (i.e. it will have units expressing an overall rate of functional change per unit time). It will predict morbidity and mortality without being dependent upon them. It will be, technically, relatively easy and inexpensive to measure. It will be equally valid in mature young adults and the very old. Finally, it will enable assertions, such as the one above, to be transformed into empirically testable hypotheses. It is hoped that this may stimulate research activity using experiments specifically designed to increase our understanding of aging mechanisms and anti-aging interventions.